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1 Journal of the Korean Ceramic Society Vol. 44, No. 6, pp. 308~31, 007. A Study on the Magnetoelectric Effect in Lanthanum Modified BiFe -PbTi Ceramics Eun Gu Lee, Sun Jae Kim,* and Jae Gab Lee** Department of Advanced Materials Engineering, Chosun University, Gwangju , Korea *Department of Advanced Institute of Nano Technologies, Sejong University, Seoul **School of Advanced Materials Engineering, Kookmin University, Seoul (Received May, 007; Accepted June 11, 007) Lanthanum ƒ BiFe PbTi z w Á½ *Á ** w œw * w ù œw ** œw (007 5 ; ) ABSTRACT Ferroelectric, magnetic, and magnetoelectric effects for lanthanum modified BiFe ceramics have been investigated. The data show that magnetoelectric polarization coefficient,α P is due to a linear coupling between polarization and magnetization, and that α P is independent of dc magnetic bias and ac magnetic field. The values of α P and magnetic induced susceptibility for lanthanum modified BiFe ceramics are much larger than those of single BiFe crystal. We believe that the magnetoelectric effect is significantly enhanced by breaking of the cycloidal spin state of a long-period spiral spin structure due to randomly distributed charged imperfections. Key words : Ferroelectric, Ferromagnetic, Magnetoelectric effects 1. ƒw w ù ù ƒw w y (magnetoelectric, ME) z 1960 Astrovƒ Cr w z y w ƒ y w š. 1,) w z», w w p w ƒ š. 3) x ¾ ƒ j δp (ME Polarization coefficient, α p = ) δh Cr α P = C/Oe-m (V ME ~ 0.0 V/Oe-cm) š Neel (T N ). 4-5) ù, Cr ù. 6,7) z» (magnetoferroelectric, ) ù w.» (P s ) y (M s ) ƒ ³e Corresponding author : Eun Gu Lee eglee@chosun.ac.kr Tel : Fax : š. x w, y ü. j k e wš» p ù BiFe, BiMn, YMn» p š. 8,9) BiFe Curie (T C )ƒ 1103 K, Neel (T N )ƒ 643 K. 10) s³ (a r = 5.61 Å, a r = ) x r e p wš, (001) H w (111) C w 3z z. BiFe» p Fe 3+ w, Fe 3+ wš 6 Fe 3+ sww (001) H w ³e š. BiFe G-x ³e wš ù, ¼ cycloidal spiral (λ=60 Å) w G-x ³ e x. 11), l z ƒ spiral». r e p BiFe w BaTi, PbTi, Pb (Fe 0.5 Nb 0.5 ) š BiFe ã p p w e p w 308

2 Lanthanum ƒ BiFe PbTi z w 309 k š. 1,13) w, BiFe Fe 3+ hopping w»» w w û p w j ù La Sm m ƒw w ƒ p š BiFe spiral spin å p w j. 3,14) BiFe La ƒwš PbTi š [(Bi,La -PbTi ] p p d wš z y wš w.. x q 99% Aldrich Bi, Fe, La, PbC TiO w. BiFe (BF) La 0, 10, 0 at% ƒ w (Bi,La PbTi (PT) š BF-xPT (0. < x < 0.55) w. yw 4 ZrO yww, w 750 o C 4 w š x z s ƒ 1100 o C 1 w. BF-xPT 0.4 PT rhombohedral-tetragonal morphotropic phase boundary(mpb) š š x w. 13) z r j» j» 10 mm, Ì 0.5 mm w 10 o C g ü kv/cm ƒw w š Ag paste (Dupont 6160) w. p HP484 d w p Berlincourt d 33 meter w. - (P-E), p - (ε-e) š linear variable differential transducer(lvdt)ƒ ü modified Sawyer-Towerz d w. ƒw w (V ME ) d w» w w, H dc Oe¾ ƒw Helmoltz g w, H ac Oe y g ƒw. r (V ME ) lock-in amplifier w H ac y d w d q 10 3 Hz. (ME voltage coefficient, α E ) V ME H ac r Ì (t) ù V/Oe-cm w. y y w, δp δe α p = = Kε o » K š δh δh ε o permittivity. w (P), P= V Kε o ME l w. w, y- š t (DC M-H hysteresis loop) p SQUID magnetometer (Quantum Design, XL7) w 5K 300 K d w. Fig. 1. Dielectric properties of (Bi,La as a function of PbTi content for (a) 0% La and (b) 0% La substitution. 3. š La ƒƒ p e w w» w 0, 0% La ƒw BF-xPT r y d w Fig. 1 ùkü. 1kHz d w La ƒw r 0.3 PT 330 ùkü. 0% La ƒw r 0.3 PT 750 d š, 0.45 PT ùkü., La ƒ ƒ ƒ PT ƒ g, p w g. Fig. BF-0.3 PT r y y ùkü. Fig. 0% La ƒw r Curie (T c ) o C. La ƒ ƒw ƒwù š T c ƒ w kƒ ù diffuse p y w. BF-0.3 PT r P-E ε-e š Fig. 3 ùk ü. Fig. 3(a) 0% La r 44«6y(007)

3 310 Á½ Á Fig.. Dielectric properties of (Bi,La as a function of temperature for 10% and 0% La substitution measured at 1 MHz. w š p š ù, 10% La ƒw r yw š p š w ƒƒ 30 µc/cm 40 kv/cm. 10% La ƒw BF-0.3 PT r ε-e š, Fig. 3(b) p Berlincourt d 33 meter d w 130 pc/n. BF w ƒ j j» w (H c ) p z» û w (ρ). w La ƒw BF-PT š w w 40 kv/cm, w 10 1 Ω-cm. Fig. 4 10% La ƒw BF-0.3 PT r, z y ùkü. Fig. 4 ƒw x ƒw, z ƒ w., w» y p, - y y wš., z (α E ) Fig. 4»» w w ƒƒ mv/oe-cm. w, 375 w w (α p = δe Kε ) ƒƒ C/Oe-m o δh ƒw ( Oe) (0.8-1 Oe) y w. y ƒ j z š Cr 10 j. Magnetoelectric coupling coefficient(k me )»» (1). α p c α p εe k me = = = µh εµ K (1) Fig. 3. P-E and ε-e hysteresis loops for (Bi,La - 0.3PbTi (a) P-E curve and (b) ε-e curve Fig. 4. Magnetic field induced electric field as a function of ac magnetic field for (Bi 0.9 w wz

4 Lanthanum ƒ BiFe PbTi z w 311 Fig. 5. Magnetoelectric coefficient (α P ) as a function of temperature for (Bi 0.9» K š c. 375 w w k me % La ƒw BF-0.3 PT r y α P y ùkü Fig. 5 α P 100 o C¾ w ƒw š 00 o C¾ š. 10% La ƒw BF-xPT r PT y y d w Fig. 6 ùkü. Fig. 6 PT ƒ ƒw α P ƒw 0.45 PT C/Oe-m. 0. PT C/Oe-m 0.45 PT 50% 0. PT r w f ƒw 50 kv/cm û š. La ƒ z d w» w 0% La ƒ BF-0.3 PT r d w Fig. 7 ùkü. Fig. 7 Fig. 6 (Bi 0.9 La 0.1 r w Fig. 7. Magnetoelectric coefficient (α P ) as a function of ac magnetic field for (Bi 0.8,La 0. ceramics measured under different dc magnetic field. ƒw ù ƒw ƒw w., ùkû. w y š w w. BiFe (Bi 0.9 La 0.1 r y p (M-H hysteresis loop) d w Fig. 8 ùkü. Fig. 8 p û» (susceptibility) š ù, (Bi 0.9 La 0.1 r j y š. ƒ Oe 5K 300 K d w y ƒƒ 0.5, 0.5 emu/cc š w ƒƒ 1.,.6 koe. La ƒ BF-PT š p y p š. Fig. 6. Magnetoelectric coefficient (α P ) as a function of ac magnetic field for various PbTi content. Fig. 8. M-H loops for BiFe single crystal and (Bi «6y(007)

5 31 Á½ Á w l ùkù cycloidal v ƒ š w w spiral v ƒ y w w. w š ùkù z Landau-Ginzburg(LG) 15,16) l w. F=F L +F exch +F an +F m ()» F L x magnetoelectric coupling(lifshitz invariant), F exch inhomogeneous exchange energy, F an magnetic anisotropy energy, F m magnetic energy. š w ww» w F an w o» ( K u = 8.5x10 ) š 3 Jm 3 w w» ( K) š w w. K y w(q) (P z ) y w ù ƒ w K=QP z tx, (3). 17) QA = πP z α eff», α eff z ³ š A stiffness. w w w α eff ù, š ey w w w spiral w v ƒ w š z ƒ ùkù. 4. La ƒ (Bi,La š La ƒ š w yw p š š, š kƒ ù diffuse p š, Cr w 10 j z ùkü. w y x y w w. w x BiFe ùkù spiral v w z ƒ ù (Bi,La š w w» q z ƒ ùkù w. Acknowledgment 006 w. (3) REFERENCES 1. D. N. Astrov, The Magnetoelectric Effect in Antiferromagnetics, Sov. Phys. JETP (1960).. D. N. Astrov, Magnetoelectric Effect in Chromium Oxide, Sov. Phys. JETP (1961). 3. H. Schmid, Multi-ferroic Magnetoelectrics, Ferroelectrics (1994). 4. V. J. Folen, G.. T. Rado, and E. W. Stalder, Anisotropy of the Magnetoelectric Effect in Cr, Phys. Rev. Lett (1961). 5. B. Krichevtsov, V. Pavlov, R. Pisarev, and V. Gridnev, Spontaneous Non-reciprocal Reflection of Light from Antiferromagnetic Cr, J. Phys.: Condensed Matter (1993). 6. E. Kita, DC Magnetoelectric Effect Measurements by a Squid Magnetometer, Ferroelectrics (1994). 7. Y. Popov, D. Belov, G. Vorob ev, A. Kadomteseva, M. Lukina, A. Zvezdin, and M. Tegeranchi, Magnetoelectric Effect and Magnetic Phase Transitions in (Fe x Cr 1-x ) Single Crystals, Sov. Phys. JETP (1996). 8. N. A. Hill, Why Are There So Few Magnetic Ferroelectrics, J. Phys. Chem., B (000). 9. V. A. Isupov, Nonlinearity of the Concentration Dependence of the Curie Temperature in Ferroelectric Perovskite Solid Solutions, Phys. Status Solidi, A (000). 10. P. Fischer, M. Polomska, I. Sosnowsa, and M. Szymanski, Temperature Dependence of the Crystal and Magnetic Structures of BiFe, J. Phys. C (1980). 11. I. Sosnowska, T. P. Neumaier, and E. Steichele, Spiral Magnetic Ordering in Bismuth Ferrite, J. Phys. C (198). 1. M. M. Kumar, S. Srinath, G. S. Kumar, and S. V. Suryanarayana, Spontaneous Magnetic Moment in BiFe - BaTi Solid Solutions at Low Temperatures, J. Magn. Magn. Mater (1998). 13. G. A. Smolenskii and V. M. Yudin, Weak Ferromagnetism of Some BiFe -Pb(Fe 0.5 Nb 0.5 ) Perovskites, Sov. Phys.- Solid State , (1965). 14. A. V. Zalesskii, A. A. Frolov, T. A. Khimich, and A. A. Bush, Composition-induced Transition of Spin-modulated Structure into a Uniform Antiferromagnetic in a Bi 1-x La x Fe System Studied using 57 Fe NMR, Sov. Phys.-Solid State , (003). 15. I. Sosnowska, M. Loewenhaupt, W. David, and R. Ibberson, Investigation of the Unusual Magnetic Spiral Arrangement in BiFe, Physica B (199). 16. B. Ruette, S. Zvyagin, A. Pyatakov, A. Bush, J. Li, V. Belotelov, A. Zvezdin, and D. Viehland, Magnetic-fieldinduced Phase Transition in BiFe Observed by High-field Electron Spin Resonance: Cycloidal to Homogeneous Spin Order, Phys. Rev. B (004). 17. A. P. Levanyuk and A.S. Sigov, Defects and Structural Phase Transitions, Gordon and Breach Science Publishers, New York, w wz